Thanks for your reply and information.
I have spoken with one of the owners. He is very aware of the swallows, and the mosquitoes they consume. He tells me that the screening of sand and soil on the bank at the rear of the sheer face where the swallows are, is continuing, but he will not be excavating any more sand until "mid to late summer". It is to be hoped that the young swallows will have fledged and gone by then. There is no sheer face at the back of the nest site, so hopefully the screening of soil and sand will not interfere with the swallows.
What is inconvenient about this situation is that the sheer face where the nests are appears to have been created in early Spring, and if no excavation had been done then, there would not have been the sheer 'cliff wall' of sand, and the swallows would not be there !!!
I see from the MNR site that they have some responsibility for protection of birds and nest sites, which was news to me. I may contact them, since my call to the Brampton Guardian. has gone unanswered.
Thanks again,
Gerry T.